Bleaching of cellulosic materials



Patented Jan. 24, 1939 UNITED STATES PATENT OFFICE 2,145,062 BLEACHING0F CELLULOSIO MATERIALS Maurice C. Taylor and James F. White, NiagaraFalls, N. Y., assignors to The Mathieson Alkali Works, Inc., New York,N. Y., a corporation of Virginia 5 No Drawing. Application April 1''],1930, 5 Serial No. 445,196 1 6 Claims. (Cl. 8-108) This inventionrelates to improvements in the material or without prejudice to thebleaching bleaching of cellulosic materials, more particularly ofmaterials composed of cellulose or'of cellulose derivatives. Theinvention involves, broadly, a novel application of the salts ofchlorous acid for this purpose.

Chlorous acid, HC1O2, forms salts with the alkali-metals, such as sodiumchlorite, NaClOz, and the alkaline-earth-metals, such as calciumchlorite, Ca(ClO2) 2.

These salts, chlorites, as applied to the bleaching of cellulosicmaterial, difier in important respects from the hypochlorites. Thehypochlorites are among the agents more commonly used for the bleachingof cellulosic materials and they are effective agentsfor this purpose.The hypochlorites, however, tend to exert a destructive action upon thecellulosic material itself. This tendency has involved, in one aspect,critical control of the bleaching operation in which the hypochloritesare used, and, in another aspect, reduction of the strength of thecellulosic material as well as actual loss of the cellulosic materialitself.

We have discovered that the chlorites in an acid aqueous solution areefiective bleaching agents but that, with the chlorites as distinguishedfrom the hypochlorites, any tendency to exert such destructive actionupon the cellulosic material itself either does not exist or is, forpractical purposes, negligible. Consequently the application of thechlorites to the bleaching of cellulosic material involves neithernecessity for critical control nor loss of or damage to the cellulosicmaterial itself. In this application of the chlorites as distinguishedfrom the hypochlorites, factors such as temperature, concentration andacidity either are not critical or are much less critical.

The bleaching of cellulosic material with the chlorites may be carriedout under acid conditions without destructive action on the cellulosicmaterial. The temperature is not critical; in general increasing thetemperature accelerates the bleaching action. The bleaching ofcellulosic material with the chlorites may be carried out attemperatures approximating 100 C. without destructive action on thecellulosic material and without appreciable loss of available oxygen.Higher temperatures may be used by carrying outthe operation undersuperatmospheric pressure. Since the chlorites are so inert with respectto cellulosic material the initial and final concentrations of thechlorite may be high or low, without destructive action on thecellulosic operation. The period of contact between the chlorite and thecellulosic material also is not critical. It is important thatsuflicient time be allowed for the bleaching operation to proceed 5 'tothe desired point but the period of any continued contact beyond thispoint is unimportant. The bleaching operation may thus be carried outunder the conditions best meeting the economic requirements in eachparticular'case. 10

According to our invention, cellulosic material is bleached bysubjection to treatment with an aqueous solution containing a chlorite,a chlorite of the alkali-metals or a chlorite of thealkalineearth-metals for example. This improved meth- 0d of bleaching isapplicable to; cellulosic materials generally, as will be apparent fromthe following examples:

Woon PULP The improved bleaching method of our invention is applicablegenerally to wood pulps intended for the manufacture of paper, includingwood pulps produced by the kraft process, by. the soda process and bythe sulfite process.

Hitherto, chlorine or a hypochlorite has usually been used in thebleaching of kraft pulp, but the use of these agents has usuallyinvolved appreciable losses of the cellulosic material of the pulpitself and a reduction of the strength of the pulp product. By the useof chlorites, instead of chlorine or hypochlorites, these difficultiesare avoided although good colors are obtained. Moreover, when chloritesare used, these difficulties do not appear even though an excess of thebleaching agent be used or even though the temperature at which theoperation is carried out be permitted to vary widely.

In applying the chlorites to the bleaching of kraft pulp, for example,the pulp is with advan 40 tage subjected to treatment with an aqueoussolution containing a chlorite, sodium chlorite or calcium chloritefor'example, having a pH value approximating 3.4-5.0 at a temperatureapproximating 80-90 C. However, the temperature at which the operationis carried out may be lower or higher than the range indicated. Theoperation may be carried out over a temperature range including 20 C.,C., C., C., etc. The concentration of the treating solution may varywidely as the salts are very soluble. The density of the pulp subjected.to treatment also may vary widely. The treatment also may be carried outin one stage or in a number of successive stages and it may be combinedwith other bleaching treatments. Successive bleaching treatments with achlorite may with advantage be combined with an intermediate treatmentwith an aqueous solution of an alkaline hydroxide such as sodiumhydroxide or of a peroxide such as hydrogen peroxide or sodium peroxideor of an acid such as hydrochloric acid or an acid salt such as sodiumacid sulfite or sodium acid sulfate. Bleaching treatments with achlorite may also be combined with preceding or succeeding treatmentwith chlorine or a hypochlorite or successive bleaching treatments witha chlorite may be combined with an intermediate treatment with chlorineor a hypochlorite.

The application of the bleaching method of our invention to wood pulpsproduced by the kraft process will be illustrated by the followingexamples:

Example I First step.2500 parts or disintegrated kratt pulp (bone dryweight) together with sufiicient water to bring the pulp density toabout 5% is heated to 80 C. 152 parts of 50% NaOH, 570 parts of glacialacetic acid and 2000 parts of water are then added to the pulp. Afterthe pulp mixture is again brought to 80 C., 231 parts of calciumchlorite and 4500 parts of water are added to the pulp mixture. Thispulp mixture is maintained in agitation at a temperature approximating80 C., for three hours. The bleach liquor is then separated fromthe pulpand the pulp is washed.

Second step-Water is then added to the washed pulp to bring the pulpdensity to about 4 50 parts of 50% NaOH are added, and this pulp mixtureis maintained in agitation at a temperature approximating 17-20'C., forten minutes. The alkaline solution is then separated from the pulp andthe pulp is washed.

Third step.The first step is then repeated except that the NaOH andacetic acid are added before the pulp is heated to the bleachingtemperature and only 154 parts of calcium chlorite, instead of 231parts, are used.

Example II is separated from the bleach liquor and is washed.

Second step.-The washed pulp is brought to a pulp density of about 5% bythe addition of water and 10 parts of 35% HCl and 150 parts of calciumchlorite are added. The pulp mixture is maintained at 65 C., for threehours. The bleach liquor is separated from the pulp and the pulp iswashed.

Example III First step-2500 parts of disintegrated kraft pulp togetherwith sufiicient water to bring the pulp density to about 5% is heated to80 C. 10 parts of 35% H] are first added and 231 parts of calciumchlorite together with 4500 parts of water are then added and the pulpmixture is maintained in agitation at a temperature approx-' imating 80C., for two hours. The bleach liquor is separated from the pulp and thepulp is washed.

Second step.-Same as in Example 1. Third step.-The washed pulp isbrought to a pulp density of about and 25 parts of 35% HCl and 154 partsof calcium chlorite are added. The pulp mixture ismaintained inagitation at a temperature approximating 80 C., for two hours. Thebleach liquor is separated from the pulp and the pulp is washed.

Example IV First step.Same as the first step in Example I except thatthe pulp mixture is maintained in agitation at a temperatureapproximating 90 C., or a somewhat higher temperature, for one andone-half hours.

Second step.-The washed pulp is brought to a pulp density of about 5% bythe addition of water. 250 parts of NaHSOa are added and the pulpmixture is maintained at a temperature approximating 17-20 C., for onehour. The sulfite solution is separated from the pulp and the pulp iswashed.

Third step.--Arter bringing the washed pulp to a pulp density of about5% by the addition of -water, it is heated to a temperature of 80 C.,

The pulp mixture is maintained in agitation at a temperatureapproximating 90 C., or a somewhat higher temperature, for one andone-half hours.

' The bleach liquor is separated from the pulp and the pulp is washed.

' Example VI First step-2500 parts of disintegrated kraft pulp togetherwith suflicient water to bring the pulp density to about 5% is heated toC., 250 parts of glacial acetic acid are added, the pulp mixture is thenheated to 50 C., and 231 parts of calcium chlorite together with 2500parts of water are added. This pulp mixture is maintained in agitationat a temperature approximating 50 C., for one and one-half hours. Thebleach liquor is separated from the pulp and the pulp is washed.

' Second step-Same as the second step in Example V.

Third step.Same as the first step in this example except that 154 partsof calcium chlorite, instead of 231 parts, are used.

Example VII First tep.--Same as the first step in Example I except thatthe entire operation is carried out at a temperature approximating 17-20C., and is continued for twenty-four hours, instead of three hours.

Second step.The washed pulp is brought to a pulp density of about 5% bythe addition of water, 250 parts of NaHSOa are added and the pulpmixture is maintained at a temperature approximating 17-20 C., forsixteen hours. solution is separated from the pulp and the pulp iswashed.

Third step.-Same as the first step in this example except that 154 partsof calcium chlorite, instead of 231 parts, are used and the bleachingoperation is continued for only nineteen hours, instead of twenty-fourhours.

It is intended and will be understood that the invention is not limitedto the foregoing examples; they are illustrative merely. An equivalentamount of sodium chlorite, for example, may be substituted for calciumchlorite in any of these examples. The pulp density, again for example,may vary from less than 445% to 10-12% or more.

In applying chlorites to the bleaching of soda pulp or sulfite pulp, forexample, the pulp is similarly subjected to treatment with an acidaqueous solution containing a chlorite, sodium chlorite or calciumchlorite for example. Neither the concentration of the solution, withrespect to the chlorite, nor the density of the pulp is critical. Thebleaching treatment with a chlorite may be carried out in successivetreatments and may be combined with other treatments as described abovein connection with kraft pulp.

The application of the bleaching method of our invention to wood pulpsproduced by the soda process will be illustrated by the followingexample:

Ercample VIII First step.-15 parts of 35% HCl and 231 parts of calciumchlorite together with 4000 parts of water are added to 2500 partsofdisintegrated soda pulp (bone dry weight) previously brought to a pulpdensity of about 5% by the addition of water. The pulp mixture is heatedto 50 C., and maintained in agitation at a temperature approximating 50C., for four hours. The bleach liquor is separated from the pulp and thepulp is washed.

Second step.Sufilcient water is added to the washed pulp to bring thepulpdensity to about 5%, 125 parts of NaHSOa are added, and the pulpmixture is maintained in agitation at a temperature approximating 17-20C., for sixteen hours. The sulfite solution is separated from the pulpand the pulp is washed.

Third step-Same as the .first step in this.

example except that 154 parts of calcium chlorite, instead of 231 parts,are used.

In bleaching soda pulp, it is advantageous to subject the pulp totreatment with an aqueous solution of an acid agent, hydrochloric acidor sodium acid sulfate for example, prior to treatment with the aqueoussolution of the chlorite. For example, 2500 parts of distintegrated sodapulp (bone dry weight) together with sufiicient water to bring the pulpdensity to about 5% may be so treated with 15 parts of 35% H] at atemperature approximating 17-20 C., for sixteen hours.

The application of the bleaching method of our invention to wood pulpsproduced by the sulfite process will be illustrated by the followingexample:

- Example IX 2500 parts of disintegrated sulfite pulp (bone dry weight)together with suflicient' water to bring the pulp density to about isheated to a temperature of 50 C., or a somewhat higher temperature.parts of 35% HCl are first 'added and 231 parts of calcium chloritetogether with 4000 parts of water are then added. The pulp mixture ismaintained in agitation at a temperature approximating 50 C., or asomewhat higher temperature, for five hours, or a somewhat shorterperiod.

Rag pulp can be bleached satisfactorily by Example X 500 parts of bluerag pulp (air dry weight) are treated with 31 parts of calcium chloriteand 11.5 parts 'of 35% HO] together with 400 parts of water. The pulpmixture is gradually raised to a temperature approximating 100 C.,during the course of two and one-quarter hours and maintained at thattemperature for one and onequarter hours, replacing water evaporatedduring the bleaching operation. The bleach liquor is separated from thepulp and the pulp is washed.

,Cor'ron' TEXTILES In the bleaching of cotton textiles, according to ourinvention, the cotton material is subjected to treatment with an acidaqueous solution containing a chlorite, sodium chlorite or calciumchlorite for example. The bleaching operation may be carried out inconjunction with the kier boiling operation by adding the chlorite tothe solution used in the boiling operation. A considerable economy withrespect to time, equipment and labor may be so efiected.

RAYON Hitherto the bleaching of rayon has usually been effected bytreatment with aqueous solutions of sodium hypochlorite; As commonlypracticed, this bleaching operation has been critical to an extreme withrespect to factors including concentration, alkalinity and temperature.We have discovered that equally satisfactory bleaching of rayon may beeffected by treatment with aqueous solutions containing a chlorite,sodium chlorite or calcium chlorite for example, in an operation which,if critical at all, is much less critical. The concentration of thetreating solution is not important, the temperature at which theoperation is carried out may vary widely, from 20 C. to more than 100C., for example, and the acidity of the treating solution may also varywidely. At low temperature, the treating solution may have, for example,a pH value approximating 1-3 and at higher temperature the solution maybe less acid. The bleaching operation may with advantage be carried outby impregnating the rayon with a neutral or alkaline aqueous solutioncontaining the chlorite and then subjecting the rayon while impregnatedwith this chlorite solution to treatment with an aqueous solution of anacid agent, such as hydrochloric acid or, if the chlorite used is achlorite of an alkali metal, sodium acid sulfate. Rayon may thus bebleached in cake form, an operation impractical with conventionalbleaching agents because of tendencies toward local fibre damage.

The application of the bleaching method of our invention to rayon willbe illustrated by the following examples:

Example XI Rayon skeins are immersed in a solution containing 3.2 gramsper liter of sodium chlorite and 3.6 grams per liter of HCl maintainedat a temperature approximating l'7-20 C., for thirty minutes. withdrawnand allowed to drip for ten minutes, and then washed.

Example XII Rayon skeins are immersed in a solution containing 64 gramsper liter of sodium chlorite and 1 gram per liter of phosphoric acidmaintained at a tempeature approximating 17-20 (3., for thirty minutes,withdrawn and allowed to drip for ten minutes, and then washed.

Example XIII Woon SURFACES It is frequently desirable to lighten thecolor of wood surfaces, in the manufacture of veneers and furniture forexample. This may be accomplished in an advantageous manner by treatmentof the wood surface with an acid aqueous solution containing a chlorite,sodium chlorite or calcium chlorite for example. The operation may becarried out at ordinary temperature with a solution containing about 160grams per liter of sodium chlorite and up to 1' gram per liter of aceticacid, for example. After drying, the strength of the wood is affectedlittle if at all. The bleach extends beneath the surface far enough tomaintain the desired color in the finished product yet not far enough toinjure the wood in any serious respect.

Gxourm Woon The bleaching method of our invention is applicable withadvantage to ground wood and ground wood pulp producing therefrom goodyields of good quality.

Lnmnnymc The chlorites of the metals of the class comprising thealkali-metals and the alkaline-earthmetals are particularly useful incarrying out our invention. The chlorite radical is the componentimportant with respect to our invention. It will be appreciated that thechlorite should be chosen for any particular purpose to avoid undesiredreactions such as the precipitation, if that is undesirable, of calciumsulfate through the use of calcium chlorite, rather than sodium chloritefor example, in a solution also containing a sulfate. y

We claim:

1. In bleaching cellulosic material, the improvement which comprisessubjecting the cellulosic material to treatment with an acid aqueoussolution containing a chlorite of a metal of the class comprising thealkali metals and the alkaline earth metals without destructive actionon the cellulosic material.

2. In bleaching kraft pulp, the improvement which comprises subjectingthe kraft pulp without destructive action on the cellulosic materialtemperature approximating 80-90 C.

3. In bleaching kraft pulp, the improvement which comprises subjectingthe kraft pulp to treatment with an aqueous solution containing achlorite of a metal of the class comprising the alkali-metals and thealkaline-earth-metals and an acid agent without destructive action onthe cellulosic material of the kraft pulp.

4 In bleaching wood pulp, the improvement which comprises subjecting thewood pulp without destructive action on the cellulosic material tosuccessive treatments with an aqueous solution containing a chlorite ofa metal of the class comprising the alkali-metals and thealkalineearth-metals and to a treatment intermediate said treatmentswith another aqueous solution of an acid agent.

5. In bleaching soda pulp, the improvement which comprises subjectingthe soda pulp without destructive action on the cellulosic materialfirst to treatment with an aqueous solution of an acid agent and then totreatment with an aqueous solution containing a chlorite of a metal ofthe class comprising the alkali-metals and the alkaline-earth-metals.

6. In bleaching rayon, the improvement which comprises subjecting therayon without destructive action on the cellulosic material to treatmentwith a non-acid aqueous solution containing a chlorite of a metal of theclass comprising the alkali-metals and the alkaline-earth-metals andthen subjecting the rayon while impregnated with the chlorite solutionto treatment with an aqueous solution of an acid agent.

MAURICE c. TAYLOR. JAMES F. WHITE.

